Download T HE spinal cord extends the whole length of the vertebral canal and

CHAPTER IV
THE NERVOUS
SYSTEM
SPINAL CORD
HE spinal cord extends the whole length
of the vertebral canal and ends near the end
of the tail as a thin, round thread. It
varies in thickness and shape in cross section, being
nearly always elliptical, but at places approaching a circle. Large, spindle-formed thickenings of
about equal diameter are present in the cervical
and lumbar regions.
A cauda equina is absent in the alligator, the
nerves of the large tail leaving the cord like the
intercostals.
On its ventral surface the cord has a deep
perpendicular fissure, the fissura ventralis, that
extends almost to the center; it extends even
along the reduced region in the tail. A vascular
membrane extends into this fissure.
A shallow but distinct furrow extends along
the dorsal side of the cord, parallel to which, on
either side, is a fine, linear furrow.
The first two spinal nerves have no dorsal roots.
T
131
132
The Alligator and Its Allies
BRAIN
The cervical cord passes insensibly into the
medulla, the dorsal furrow becoming wider and
more shallow as it merges into the fourth ventricle.
A dorsal view of the brain is shown in Figure 30,
A. The most prominent structures here seen are
the cerebral hemispheres, VH, whose combined
transverse diameter is greater than their longitudinal. The tapering, cephalic end of each hemisphere forms an olfactory tract, I, which extends
cephalad to form the olfactory bulb, B. ol. Lying
between the caudal ends of the hemispheres is a
small conical body, G.p., called by Bronn and
others the pineal body. The writer has found (62),
however, that this body is the paraphysis rather
than the epiphysis. Caudad to the cerebrahemispheres and in contact with them are the
optic lobes, MH; they have about the same
shape and position as in the frog, but are much
smaller in proportion to the size of the hemispheres.
Immediately caudad to the optic lobes is the cerebellum, HH, somewhat elliptical in outline as seen
from above.
Extending caudad from beneath the cerelellum
is the medulla, NH, with its triangular fourth
ventricle. The outlines of the medulla are somewhat obscured by the numerous roots of the
eighth to eleventh cranial nerves, VIII-XI, which
arise along its dorsal border. The medulla, as was
A
'A
f-ol
-X
C
A
Rif
1711
FIG. 30.
BRAIN OF ALLIGATOR.
C, lateral view.)
(A, dorsal; B, ventral; and
(From Wiedersheim, slightly altered.)
VH, cerebral hemispheres, each of which gives rise postero-laterally to a
hippocampal lobe partially overlying the corresponding optic tract,
Tr.opt; ZH, thalamencephalon; MH, optic lobes; HH, cerebellum;
NH, medulla oblongata; I-XII, cranial nerves; I, 2, first and second
spinal nerves; B.ol, olfactory bulb; Tro, olfactory tract; G.p, paraphysis; Jnf, infundibulum; Hyp, hypophysis; Med, spinal cord.
The Nervous System
133
said above, passes, as is usually the case, without
any line of demarcation into the spinal cord, the
obex filling in the apex of the fourth ventricle at the
anterior end of the median dorsal fissure.
A lateral view of the brain is shown in Figure
30, C. The hemisphere, VH, is conical in outline,
with a small projection from the posteroventral
region; its continuation forwards as the olfactory
tract, Tro., and bulb, B. ol., is plain. Beneath it
and extending forwards are the prominent optic
nerve, II, and tract. Caudad to the latter and
projecting ventrad and caudad are the infundibulum, Inf., and hypophysis, Hyp.
Caudad to the cerebrum are seen the end of the
paraphysis, G.p., the optic lobes, MH, and the
cerebellum, HH. From the cerebral peduncles
(ventrad to the optic lobes) arises the oculomotor
nerve, III, and dorsocaudad to this, between
the optic lobe and the cerebellum, arises the
trochlear nerve, IV. From the middle zone (in a
dorsoventral direction) of the medulla, ventrad to
the cerebellum, arises the very large trigeminal
nerve, V; while from its usual place, on the ventral
surface of the medulla, the abducens nerve, VI,
takes its origin by several roots. At some distance caudad from the trigeminal, from the dorsal
surface of the medulla, as noted above, the very
large acoustic nerve, VIII, arises; and immediately
ventrad to this, on the side of the medulla, the
facial nerve, VII, may be seen. Commencing just
134
The Alligator and Its Allies
caudad to the acoustic and extending along the
upper border of the medulla and beginning of the
spinal cord, are seen a dozen or more small nerve
roots, which unite to form the glossopharyngeal, IX,
vagus, X, and spinal accessory, XI, nerves. Ventral to the roots of the last, on the ventral surface
of the medulla, arise the roots of the hypoglossal
nerve, XII. A short distance caudad to this nerve
are seen the first two spinal nerves, I and 2, which
have, as noted above, no dorsal roots.
A ventral view of the brain is shown in Figure 3o,
B. The cerebral hemispheres, VH, have the same
outline, of course, as in the dorsal view, but the
rounded projection from the caudal end of each
is here seen on each side of the infundibulum, Inf.
The infundibulum is in close contact with the
chiasma anteriorly, and lies close between the
converging optic tracts, Tr. opt. From the chiasma the optic nerves, II, extend, in an anterolateral direction, almost at right angles to each
other. The appearance of the olfactory tracts, I,
is the same as in the dorsal view. Caudad to the
infundibulum, from the cerebral peduncles, ZH,
arise the rather small oculomotor nerves, III.
Caudad to these, from near the ventral fissure,
on the middle region of the medulla, arise the abducens nerves, VI, and from the ventral side of the
posterior part of the medulla and of the anterior
end of the cord arise the hypoglossal, XII, and
the first two spinal nerves, I and 2. The ori-
The Nervous System
135
gins of the other cranial nerves were described in
connection with the lateral view of the brain, where
they show more clearly. On each side of the cerebral peduncles is seen the ventrolateral edge of
the corresponding optic lobe. The pyramidal
tracts are seen, extending caudad from the region
of the peduncles, as a swelling on each side of the
median ventral fissure.
THE CRANIAL NERVES (CROCODILE)
The origin of each of the cranial nerves was described in connection with the lateral and ventral
views of the brain. A full description of the
distribution of these nerves would require more
space than the limits of this book will allow, but
a brief account will now be given.
I and II. The olfactory and optic nerves.
These two large nerves go immediately to their
respective sense organs, so that no further discussion of them need be here given.
III. The oculomotor nerve. The single stem
divides into three branches: a median, going to
the externus rectus muscle; a lateral, going to the
inferior rectus muscle; and an intermedial, going
to the inferior oblique muscle.
IV. The trochlear (pathetic) nerve leads to
the superior oblique muscle.
V. The trigeminal nerve. The distribution
of this nerve is very complicated. It has three
136
The Alligator and Its Allies
main divisions: (I)
the ophthalmic branch, (2)
the superior maxillary branch, and (3) the inferior
maxillary branch. (i) The ophthalmic in turn
divides into two branches: the smaller, frontal,
going to the integument of the upper and lower
eyelids; the larger, nasal, going chiefly to the nasal
cavity but also sending some small branches to the
upper and lower eyelids. (2) The superior maxillary branch separates into a number of divisions:
(a) a branch that, in the neighborhood of the
auditory capsule, fuses with the facial nerve; (b)
a twig to the integument of the forehead and
to the upper and lower eyelids; (c) a branch to the
Harderian gland and the conjunctiva; (d) a branch
to the neighborhood of the cheek, to the angle of
the mouth, and to the palatine branch of the facial
nerve; (e) a branch to the palate; (f) a branch to
the integument of the upper jaw; (g) a branch to
the teeth of the upper jaw. (3) The inferior
maxillary branch divides into four branches: (a)
this division supplies the skin of the cheek
region; (b) a branch to the chewing muscles; (c)
a branch that divides into two nerves-the
first going to the skin of the lower jaw, the
second dividing again into two nerves, both of
which lead to the integument of the lower jaw;
(d) the fourth division of the inferior maxillary,
known as the inferior alveolar, itself divides into
two twigs-(a') the first twig divides into two
parts, a larger and a smaller, both of which lead,
The Nervous System
137
by different paths, to the inner skin of the
mouth; (b') the second twig divides into four
parts-two leading to the mylohyoid muscle
and to the integument at the corner of the
mouth, one to the integumental glands at the
corner of the mouth, and one to the floor of
the mouth cavity.
VI. The abducens nerve leads to the retractor
oculi muscle and to the muscle for the nictitating
membrane.
VII. The facial nerve gives off three main
branches: (I) the first divides again into three
twigs-(a) connecting with a branch of the trigeminal nerve, (b) and (c) connecting with the trigeminal and also leading to the palate; (2) the second
branch divides into two twigs that connect with
the glossopharyngeal nerve; (3) the third branch
divides into two parts, a muscular twig, and the
chorda tympani.
VIII. The auditory or acoustic nerve leads, of
course, to the sensory regions of the ear.
IX. The glossopharyngeal nerve divides into
four main branches, as follows: (I) to the larynx,
(2) to the oesophagus, (3) to the hyomaxillary
and sterno-maxillary muscles, and (4) to the tongue.
There are also certain communicating twigs with
the facial and vagus nerves.
X. The vagus or pneumogastric nerve gives
off four branches: (I) and (2) communicate with
each other and supply the pharynx, larynx, oesoph-
138
The Alligator and Its Allies
agus, and trachea; (3) goes to the oesophagus; (4)
goes to the heart, lungs, and stomach.
XI. The spinal accessory nerve. There seems
to be some doubt as to the exact identity and
distribution of this nerve, but Bronn says that,
according to Fischer, it gives twigs to the lower
head-muscles and then divides into fine branches
in the atlanti-mastoideus muscle.
XII. The hypoglossal nerve, going to the region
of the tongue, divides into three branches: (i) the
median and smallest goes to the sterno-maxillary
muscle; (2) the inner and larger goes to the same
muscle and also to the coraco-hyoid and sternohyoid muscles; (3) the outer and largest divides into three twigs of which the first two lead to the
hyomaxillary and sterno-maxillary muscles respectively, while the third divides into two twigs that
lead to the hyoglossal and genioglossal muscles
respectively.
THE SPINAL NERVES
As was noted above, the dorsal roots of the first
two spinal nerves are lacking.
I, II, and III. The ventral branches of these
three nerves supply the smaller, ventral neck
muscles.
IV. The ventral branch of this nerve innervates
with its chief divisions the ventral muscles, the
sphincter colli, and the integument of the neck,
The Nervous System
139
and sends a small branch to the levator scapulae
superficialis muscle.
V. The ventral branch of this nerve sends
branches to the ventral muscles of the neck, to the
levator scapula superficialis; a large branch goes
to the sterno-mastoid; and the rest of the nerve
distributes itself in the sphincter colli and the
integument and ventral muscles of the neck.
VI. The sixth nerve distributes itself to the
ventral musculature and to the integument of
the neck, and sends a fairly strong branch to the
levator scapulae superficialis muscle and to the
most anterior part of the collo-thoraci-suprascapularis profundus muscle.
VII. The seventh nerve is the first to enter,
by a small branch, into the brachial plexus (Figure
31). It also sends a branch to the ventral muscles
and the integument of the neck, and three branches
to various shoulder muscles.
VIII. The ventral branch of the eighth nerve
(Figure 31) is the second largest nerve of the
brachial plexus. It gives some twigs to the ventral muscles and then gives one or two nerves to
the collo-thoraci-suprascapularis profundus and the
serratus superficialis muscles. The rest of the
nerve divides into an inferior and a superior branch
which unite with the ninth nerve.
IX. The ninth and tenth nerves are the largest
of the brachial plexus. The former, after giving
off some twigs to the ventral musculature and to
140
The Alligator and Its Allies
the serratus superficialis and the hinder regions
of the collo-thoraci-suprascapularis profundus
muscles, unites with the tenth nerve just after
giving off the small thoracicus inferior nerve
to the costo-coracoideus muscle. After uniting
with the tenth nerve the ninth nerve immediately
divides into two branches that form loops with
branches of the eighth nerve, the whole making a
very complicated plexus.
X. The tenth nerve, as noted above, is one of
the two largest nerves of the brachial plexus.
After giving off a single nerve to the ventral
musculature, this nerve unites with the eleventh
nerve; it then gives a branch to the costo-coracoideus muscle and forms a loop with the ninth nerve.
After giving off a couple of nerves it again divides
into two equal branches which unite with similar
branches of the eighth nerve.
XI. The eleventh nerve is next to the smallest
of the plexus. Besides branches to the trunk
musculature it gives a fine twig to the integument
of the axilla and unites with the tenth nerve in the
brachial plexus. This is the last nerve that enters
into the brachial plexus.
The distribution of the nerves of the brachial
plexus is as follows (Fig. 31): (a) supracoracoideus
to the muscle of that name and to the integument
of the breast; (b) thoraci inferiores nerves (ioa)a complex of nerves from the eighth, ninth, and
tenth spinal stems-lead to the costo-coracoideus
PIG. 31. BRACHIAL PLEXUS OF C. AcuTUS. (From Bronn, after F-irbringer.)
anterior
21muscular
22. coraco-braVII-X.
ventral
branches of
chialis.
seventh to
22c. branchforthe
el
distal
belly
spie venth
na
of biceps
muscle.
nerves.
30. thoracicus
24. muscular
branch for
antror
VII.
FI
thehumeroantebrachi4. thoracicus
7.
7a.
superior V.
thoracicus
superior
supnterior
VI.
proximallyleading
thoracicus
cutaneus
(25+42).alisinferior.
brachii and
branch
for.
ante2brachii
medialis.
29. subscapularis.
distally-leading thoracicus superior
VII.
thoracicus
superiorX .
31.
o10a
Ioa, Ioai,
2,IOa 3.
thoracicus
33.
34.
7b.
9.
18.
19.
21.
inferior.
cutaneuspectoralis.
pectoralis.
brachialis
Slongus inferior.
p.--
32.
36.
36a.
dorsalis scapule (post3e rior).
cutaneusbrachi superior laterais.
indeltoides
ferior.
brachialis
longus superior.
anconeus.
scapulo-humeralisprofundus.
C1_111-,
I
The Nervous System
141
muscles and to the anterior part of the transversus
abdominis muscle; (c) the pectoralis (19), a large
nerve leading to the muscle of that name; (d)
cutaneus pectoralis (18), fine branches from the
XIth spinal nerve to the integument of the axilla
and the neighboring parts of the breast; (e) coracobrachialis (22) to the like named muscle; (f) cutaneus brachii et antebrachii medialis (25 + 42) to
the medial side of the integument of the upper
and fore arm; (g) brachialis longus inferior (21), a
large nerve that supplies the biceps and humeroantebrachialis inferior muscles, and then divides
into the medianus and ulnaris inferior nerves; (h)
subscapularis (29) to the like named muscle; (i)
scapulo-humeralis profundus (36a) to the like
named muscle; (j) axillaris, a large stem that
divides into two main twigs that lead to the skin
of the lateral side of the upper arm, to the proximal
part of the forearm, to the humero-radialis muscle,
and to the deltoides coraco-sternalis muscle; (k)
dorsalis scapula (posterior) (31) to the deltoideus
scapularis muscle; (1) teres major (29b), one
(alligator) or two (crocodile) middle-sized nerves
to the teres major muscle; (m) latissimi dorsi (29b)
to the like named muscle; (n) brachialis longus
superior (radialis) (not shown in Figure 31) to the
extensor side of forearm and the hand.
Of the spinal nerves between the brachial and
crural plexuses Bronn gives no description for the
Crocodilia. The most posterior nerve of the
142
The Alligator and Its Allies
P
8
4 zVI
qiaa.r
a
FIG. 32.
CRURAL
"
b
PLEXUS AND ISCHIADIC PLEXUS OF THE LEFT
THE NERVE BRANCHES ARE SHOWN AS
SIDE OF A. MISSISSIPPIENSIS.
THE CRURAL PLEXUS
FAR AS THEIR ENTRANCE INTO THE MUSCLES.
THE OBTURATOR NERVE
IS MADE UP OF THE PRESACRAL STEMS a, b, c.
Is BUILT OF TWO BRANCHES FROM STEMS A & B.
GADOW.)
presacral nerves.
postsacral nerves.
sacral nerve (26th
spinal nerve).
to extensor ileo-tibialis muscle.
to femoro-tibialis muscle.
to ileo-fibularis muscle,
to ileo-femoralis muscle.
to caudi-ileo-femoralis muscle.
caudi-femoralis muscle.
a, b, c.
a & f.
s=XXVI.
2.
3.
4.
5.
6.
7.
8.
9.
I1.
13.
14.
15.
(FROM BRONN, AFTER
to flexor tibialis externus
muscle.
to flexor tibialis internus
muscle.
to ischio-femoralis muscle.
inpubo - ischio - femoralis
temus.
pubo - ischio - femoralis externus muscle.
to pubo-ischio-femoralis posterior muscle.
The Nervous System
143
former plexus is the eleventh and the most anterior
nerve to take part in the latter is the twenty-third,
so that there are eleven nerves that are doubtless
distributed to the regions not supplied by the two
plexuses.
The crural-ischial plexuses (Fig. 32) are made up
of branches from five nerves, three presacral (a, b,
and c), the sacral (s= xxvi), and one postsacral (a);
the second postsacral shown in the figure apparently does not enter into the plexus.
The first and second presacrals terminate chiefly
in the abdominal and thigh muscles, though the
second sends a large branch to fuse with a branch
from the third to form the large obturator nerve
(N. obt.) that leads to the muscles of the thigh and
knee.
The third presacral sends a branch back to fuse
with the large sacral (s=xxvi), and these two,
together with a branch from the first postsacral,
form a complicated network that sends numerous
branches to the muscles of the pelvic and femoral
regions, to the skin, legs, and tail, as shown in
Figure 32. The large muscles of the tail are
innervated by the regular, metameric nerves of
that region, and since there are usually thirty-nine
caudal vertebrae, there are probably about that
many pairs of caudal nerves, although the last few
vertebrae and the muscles of that region are so
small it may be that some of the nerves are
lacking.
144
The Alligator and Its Allies
SPECIAL SENSE ORGANS
It is not possible in a work of this size to give
much space to the discussion of the anatomy of
the special sense organs. A few of the main features will be given here, taken mainly from Bronn's
Thierreich,but for details of structure the reader is
referred to that larger work.
The Eye. As might be expected, the Crocodilia
have the usual upper and lower eyelids and the
nictitating membrane. Except along their thickened rims the lids are usually rather faintly pigmented, and near the thickened border numerous
goblet cells are found.
The structure of the upper and lower lids is
similar except that in the former a bony formation
is present, as a support to that lid, even in very
young animals. The arrangement of the muscles,
which are of both smooth and striped fibers, and
the histological structure cannot be described here.
The nictitating membrane is strongly developed
in the Crocodilia. Its outer surface is marked by
two fairly high folds that are conspicuously pigmented. The cartilage described in the nictitating membrane of Lacerta is wanting, according
to Bronn, in the Crocodilia.
The glands of the eye are of three types: the
lachrymal glands proper, the Harderian glands,
and the conjunctival glands. The lachrymal gland
is small in proportion to the size of the eye. It
The Nervous System
145
is an elongated, almost band-like structure situated in the roof of the eye-socket, near its border;
its long axis lies in an antero-posterior direction.
It is so closely inclosed by and united with connective tissue that it is difficult to find.
The Harderian gland is much larger than the
lachrymal gland proper and is easily found. It
lies in the forward part of the eye-socket and is
of a somewhat three-cornered shape.
From its
outer and forward base it sends a short, delicate
duct to open between the nictitating membrane
and the eyeball.
The lachrynial canal is well developed in the
Crocodilia. Near the forward angle of the eye, on
the inner side of the lower lid, are found from
three to eight tear dots, lying in a row from behind
forward. Each of these dots opens into a small
elongated sac. This sac opens downwards and
forwards into a common canal, which canal, at
first narrow but soon widening, extends for a
time parallel to the free border of the eyelid and
then enters the opening in the hinder side of the
lachrymal bone. Rathke found none of these
tear dots on the upper eyelid so concluded that
the lachrymal fluid could escape only through
the lower lid. This canal, which might correspond
to the lachrymal sac of higher forms, is rather
narrow until it enters the lachrymal bone, then it
becomes considerably wider and forms a sort of reservoir that Rathke calls the "saccus naso-lachryma-
146
The Alligator and Its Allies
lis." This reservoir is of irregular form and opens
forwards into the base of the nasal cavity proper.
The third type of gland mentioned above, the
conjunctival, is found on the lower eyelid where
the conjunctiva passes from the lid to the eyeball.
The gland is of a "scattered acinose" type.
The usual muscles of the eyeball are found in
the Crocodilia. The four rectus and two oblique
muscles have about the usual arrangement and are
attached to the eyeball by very short aponeuroses.
The retractoroculi muscle is only weakly developed.
It consists of two separate bundles which, lying
behind the optic nerve, arise from the forward bony
wall of the socket and are inserted on the sclera
very near the optic nerve.
The eyeball consists of the usual layers, including, as might be expected from the nocturnal habits
of the Crocodilia, a typical tapetum lucidum.
SIn the sclera, instead of the bony ring common
to the saurians, is found a well-developed cartilage
covered with the fibrous layer of the sclera; the
fibers of this layer are arranged into two more or
less distinct layers.
While not worked out in detail the cornea consists
of the usual five layers.
?In the iris the musculature is less developed than
in the birds; Bronn thinks this may be compensated for by the greater development of the
"vascular structures."
The pupil is a vertical slit.
The Nervous System
147
The choroid is very closely united on the outside
with the sclera; on the inside it is less closely attached to the retina except at the ora serrata. It
consists of an outer fibrous coat, an inner, unstratified pigmented epithelium derived embryologically
from the pigmented layer of the retina, and the
ground substance which is a network of irregular
and very vascular cells.
As in probably all reptiles there is present in
the Crocodilia a vascular pigmented fold of the
choroid, the pecten, which projects into the middle
of the cavity of the eyeball.
In the retina Bronn describes the following ten
layers, which are those commonly given in other
vertebrate retinas: (i) the inner limiting membrane, (2) optic fiber layer, (3) ganglion cell layer,
(4) inner granular layer, (5) inner nuclear layer, (6)
outer granular layer, (7) outer nuclear layer, (8)
outer limiting membrane, (9) cone layer, (Io)
pigmented layer. The Crocodilia differ from probably all other reptiles in having rods as well as
cones in the retina. The rods are more numerous
except in the neighborhood of the fovea centralis
where the cones predominate; in the fovea itself
only cones are found.
The lens does not show any characteristics
unusual enough to warrant special description.
The Ear. The ear is of special interest here
because it is in the Crocodilia that are first found
the three distinct regions of the ear that are seen
148
The Alligator and Its Allies
in the Aves and Mammalia: the external auditory
meatus, the tympanic cavity, and the labyrinth.
It is the presence of the meatus that lifts the Crocodilia above the other Reptilia.
Two strong folds of integument, one above and
one below, completely cover the outer ear and
allow it to open as a mere slit on the lateral surface
of the head a little back of the corner of the
eye. By lifting the upper valve one may perceive
the lower half of the meatus and the bottom of
the tympanic membrane. The upper valve is the
larger and is sickle shaped; the lower is smaller
and more three cornered. Both spring from the
outer surface of the squamosal bone, from its
posterior obtuse angle to its anterior union with
the postfrontal. The lower fold is raised highest
behind the corer of the eye and is lost in the middle
of the rima auditoria; by this Hasse indicates the
position of the outer opening of the external auditory meatus. The form of the meatus may be
compared to a wedge whose base is directed dorsomedio-caudad and whose edge points in a ventrolatero-cephalic direction; its side walls are either
soft or bony; its outer end is covered by the folds;
at its inner end is the tympanic membrane or drum.
The drum is a round, soft, elastic membrane in
which a radial arrangement of its constituent
fibers may be seen. It is funnel shaped from without and above, and the fibers radiate from the apex
to which the columella is attached. The membrane
The Nervous System
149
is stretched taut and while it does not, as in the
higher vertebrates, lie in a bony groove, it possesses
around its periphery a strong thickening of circular
fibers, the annulus tympanicus, by means of which
it is closely united with the lining membrane of
the outer ear passage. The drum is attached chiefly
to the quadrate but in part to the squamosal bone.
The middle ear is divided into an outer part,
the tympanic cavity proper, and a part next to the
labyrinth, the recessus cavi tympani. Within the
tympanic cavity, besides blood-vessels and nerves,
is found the columella with its appendage (found
in all Reptilia), the recessus scalce tympani. The
tympanic cavity is formed mainly by the quadrate,
though the exoccipital and squamosal bones take
some part. In outline it might be compared to a
truncated, four-sided pyramid, with its base below,
its truncated apex above, and with an anterior, a
posterior, a mesial, and a lateral side.
From the floor of each tympanic cavity a
Eustachian tube leads towards the throat. These
tubes unite and connect with the throat by a single
small opening just behind the posterior nares, as
shown in the figures of the skull.
The semicircular canals with their ampulce lie in
the usual positions as seen in other vertebrates: the
anterior vertical, posterior vertical, and horizontal.
The details in structure of the inner ear cannot be
given here. The nervous epithelium is said to have
the same characteristics as in other vertebrates.